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Luc Lenain, Nicholas M. Statom, and W. Kendall Melville

Abstract

As reported in 1954, more than a half century ago, C. Cox and W. Munk developed an empirical model of the slope distribution of ocean surface waves that has been widely used ever since to model the optical properties of the sea surface and is of particular importance to the satellite remote sensing community. In that work, the reflectance of sunlight was photographed from a Boeing B-17G bomber and was then analyzed. In this paper, surface slope statistics are investigated from airborne scanning topographic lidar data collected during a series of field experiments off the coast of California and in the Gulf of Mexico, over a broad range of environmental conditions, with wind speeds ranging from approximately 2 to 13 m s−1. Unlike the reflectance-based approach of Cox and Munk, the slope distribution is computed by counting laser glints produced by specular reflections as the lidar is scanned over the surface of the ocean. We find good agreement with their measurements for the mean-square slope and with more recent (2006) results from Bréon and Henriot that were based on satellite remote sensing. Significant discrepancies for the higher-order statistics are found and discussed. We also demonstrate here that airborne scanning lidar technology offers a viable means of remotely estimating surface wind speed and momentum flux.

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Benjamin D. Reineman, Luc Lenain, Nicholas M. Statom, and W. Kendall Melville
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Leonel Romero, J. Carter Ohlmann, Enric Pallàs-Sanz, Nicholas M. Statom, Paula Pérez-Brunius, and Stéphane Maritorena

Abstract

Coincident Lagrangian observations of coastal circulation with surface drifters and dye tracer were collected to better understand small-scale physical processes controlling transport and dispersion over the inner shelf in the Gulf of Mexico. Patches of rhodamine dye and clusters of surface drifters at scales of O(100) m were deployed in a cross-shelf array within 12 km from the coast and tracked for up to 5 h with airborne and in situ observations. The airborne remote sensing system includes a hyperspectral sensor to track the evolution of dye patches and a lidar to measure directional wavenumber spectra of surface waves. Supporting in situ measurements include a CTD with a fluorometer to inform on the stratification and vertical extent of the dye and a real-time towed fluorometer for calibration of the dye concentration from hyperspectral imagery. Experiments were conducted over a wide range of conditions with surface wind speed between 3 and 10 m s−1 and varying sea states. Cross-shelf density gradients due to freshwater runoff resulted in active submesoscale flows. The airborne data allow characterization of the dominant physical processes controlling the dispersion of passive tracers such as freshwater fronts and Langmuir circulation. Langmuir circulation was identified in dye concentration maps on most sampling days except when the near surface stratification was strong. The observed relative dispersion is anisotropic with eddy diffusivities O(1) m2 s−1. Near-surface horizontal dispersion is largest along fronts and in conditions dominated by Langmuir circulation is larger in the crosswind direction. Surface convergence at fronts resulted in strong vertical velocities of up to −66 m day−1.

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Benjamin D. Reineman, Luc Lenain, Nicholas M. Statom, and W. Kendall Melville

Abstract

Instrumentation packages have been developed for small (18–28 kg) unmanned aerial vehicles (UAVs) to measure momentum fluxes as well as latent, sensible, and radiative heat fluxes in the atmospheric boundary layer (ABL) and the topography below. Fast-response turbulence, hygrometer, and temperature probes permit turbulent momentum and heat flux measurements, and shortwave and longwave radiometers allow the determination of net radiation, surface temperature, and albedo. UAVs flying in vertical formation allow the direct measurement of fluxes within the ABL and, with onboard high-resolution visible and infrared video and laser altimetry, simultaneous observation of surface topography or ocean surface waves. The low altitude required for accurate flux measurements (typically assumed to be 30 m) is below the typical safety limit of manned research aircraft; however, with advances in laser altimeters, small-aircraft flight control, and real-time kinematic differential GPS, low-altitude flight is now within the capability of small UAV platforms. Flight tests of instrumented BAE Systems Manta C1 UAVs over land were conducted in January 2011 at McMillan Airfield (Camp Roberts, California). Flight tests of similarly instrumented Boeing Insitu ScanEagle UAVs were conducted in April 2012 at the Naval Surface Warfare Center, Dahlgren Division (Dahlgren, Virginia), where the first known measurements of water vapor, heat, and momentum fluxes were made from low-altitude (down to 30 m) UAV flights over water (Potomac River). This study presents a description of the instrumentation, summarizes results from flight tests, and discusses potential applications of these UAVs for (marine) atmospheric boundary layer studies.

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W. Kendall Melville, Luc Lenain, Daniel R. Cayan, Mati Kahru, Jan P. Kleissl, P. F. Linden, and Nicholas M. Statom

Abstract

Satellite remote sensing has enabled remarkable progress in the ocean, earth, atmospheric, and environmental sciences through its ability to provide global coverage with ever-increasing spatial resolution. While exceptions exist for geostationary ocean color satellites, the temporal coverage of low-Earth-orbiting satellites is not optimal for oceanographic processes that evolve over time scales of hours to days. In hydrology, time scales can range from hours for flash floods, to days for snowfall, to months for the snowmelt into river systems. On even smaller scales, remote sensing of the built environment requires a building-resolving resolution of a few meters or better. For this broad range of phenomena, satellite data need to be supplemented with higher-resolution airborne data that are not tied to the strict schedule of a satellite orbit. To address some of these needs, a novel, portable, high-resolution airborne topographic lidar with video, infrared, and hyperspectral imaging systems was integrated. The system is coupled to a highly accurate GPS-aided inertial measurement unit (GPS IMU), permitting airborne measurements of the sea surface displacement, temperature, and kinematics with swath widths of up to 800 m under the aircraft, and horizontal spatial resolution as low as 0.2 m. These data are used to measure ocean waves, currents, Stokes drift, sea surface height (SSH), ocean transport and dispersion, and biological activity. Hydrological and terrestrial applications include measurements of snow cover and the built environment. This paper describes the system, its performance, and present results from recent oceanographic, hydrological, and terrestrial measurements.

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